Conventionally, there is known a device that applies filtering processing, such as edge enhancement or smoothing, to image data (for example, Jpn. Pat. Appln. Laid-Open Publication No. 11-220618). In this conventional technology, the filtering processing is applied to all input color image signals, and it is determined whether or not the input signal is a black character signal. When the input signal is the black character signal, a filtering signal is used to output an image with a solid black color to prevent color blurring.